1. Field of the Invention
This invention relates to an epitaxial wafer and a method of producing the same, and more particularly to an epitaxial wafer having a given gettering means suitable for an imaging device or a thin film device and a method of producing the same.
2. Description of the Related Art
As a problem in a semiconductor process is mentioned an incorporation of a heavy metal as an impurity into a silicon wafer. If the heavy metal is incorporated, it remarkably exerts adverse influences on device characteristics such as poor pose time, poor retention, poor junction leak, insulation breakage of oxide film and the like. Therefore, it is common to adopt a gettering method for suppressing the diffusion of the heavy metal into a device forming (active) region at a front surface side of the silicon wafer.
As the conventional gettering method are known an intrinsic gettering method (IG method) utilizing micro defects inside the silicon wafer as a gettering site (capture region) and an extrinsic gettering method (EG method) wherein mechanical strain is given to a surface (rear surface of the wafer opposite to the device forming surface by a sand blast process or the like or a polycrystalline silicon film is formed on the rear surface as a gettering site.
With the advance of techniques for electronic devices such as mobile phones, digital still cameras and the like, it is extending to thin the thickness of semiconductor device to be built into these electronic devices. As a result, it is demanded to develop silicon wafers in which the aforementioned gettering site is existent in a region closer to the device active layer for obtaining a high gettering ability.
However, even when the silicon wafer having the gettering site closer to the device active layer is formed by the IG method as compared with the EG method, a DZ layer having no oxygen precipitation nucleus may be formed in a region ranging from the surface of the wafer to not less than 10 μm by the heat treatment. The final thickness of the semiconductor device tends to be more thinned and will be anticipated to be about 10 μm in the year of 2010 or later. In this case, the gettering region is not existent in the wafer, so that metal impurities generated at the device step can not be gettered fully. As the impurities generated in the device active layer can not be gettered sufficiently, the IG and EG methods can not be applied to the thinned device as they are.
As the silicon wafer having the gettering site closer to the device active layer is mentioned a silicon wafer as described in JP-A-H05-152304 wherein carbon ions are implanted into a surface of a silicon wafer to form a gettering layer at a shallow position from the surface and then an epitaxial layer is grown on such an surface of the wafer. Also, there is mentioned a silicon wafer as described in JP-A-2006-216934 wherein a gettering layer containing C, Ge, Sn and/or Pb is formed by CVD method or a doping method and then an epitaxial layer is formed on the gettering layer.
In the silicon wafer produced by the method described in JP-A-H05-152304, however, it is required to use a very expensive ion implantation apparatus, and also there are problems such as metal contamination, generation of particles and the like due to the ion implantation through the ion implantation apparatus itself, as well as occurrence of defects induced in a finished epitaxial film due to the particles. In the silicon wafer produced by the method described in JP-A-2006-216934, the gettering effect is developed through a lattice stress effect introducing carbon into lattice positions, so that it is required to forming the gettering layer at a low temperature within a temperature range of 500-750° C. for introducing carbon into the lattice positions and hence the layer forming rate is largely lowered, which is unsuitable in the mass production. Since all of these silicon wafers in these patent documents are produced at the low temperature growing method, there is a problem such as the deterioration of quality due to defects, haze and the like of the device active layer as a final epitaxial film.